This invention relates to ceramic compositions for thermal barrier coatings having reduced thermal conductivity comprising zirconia and a stabilizer component having a first metal oxide such as yttria and a second metal oxide such as lanthana or gadolinia. This invention further relates to coatings prepared from such compositions, articles having such coatings and methods for preparing such coatings for the article.
Components operating in the gas path environment of gas turbine engines are typically subjected to significant temperature extremes and degradation by oxidizing and corrosive environments. Environmental coatings and especially thermal barrier coating are an important element in current and future gas turbine engine designs, as well as other articles that are expected to operate at or be exposed to high temperatures, and thus cause the thermal barrier coating to be subjected to high surface temperatures. Examples of turbine engine parts and components for which such thermal barrier coatings are desirable include turbine blades and vanes, turbine shrouds, buckets, nozzles, combustion liners and deflectors, and the like. These thermal barrier coatings typically comprise the external portion or surface of these components are usually deposited onto a metal substrate (or more typically onto a bond coat layer on the metal substrate for better adherence) from which the part or component is formed to reduce heat flow (i.e., provide thermal insulation) and to limit (reduce) the operating temperature the underlying metal substrate of these parts and components is subjected to. This metal substrate typically comprises a metal alloy such as a nickel, cobalt, and/or iron based alloy (e.g., a high temperature superalloy).
The thermal barrier coating is usually prepared from a ceramic material, such as a chemically (metal oxide) stabilized zirconia. Examples of such chemically stabilized zirconias include yttria-stabilized zirconia, scandia-stabilized zirconia, ceria-stabilized zirconia, calcia-stabilized zirconia, and magnesia-stabilized zirconia. The thermal barrier coating of choice is typically a yttria-stabilized zirconia ceramic coating. A representative yttria-stabilized zirconia thermal barrier coating usually comprises about 7 weight % yttria and about 93 weight % zirconia. The thickness of the thermal barrier coating depends upon the metal substrate part or component it is deposited on, but is usually in the range of from about 3 to about 70 mils (from about 75 to about 1795 microns) thick for high temperature gas turbine engine parts.
Thermal barrier coatings comprising yttria-stabilized zirconia are usually formed on or applied to a bond coat layer of a superalloy metal substrate, such as those used in turbine airfoils, by physical vapor deposition (PVD), such as electron beam physical vapor deposition (EB-PVD) or plasma spray, such as air plasma spray (APS) techniques. Thermal barrier coatings deposited by EB-PVD techniques have a columnar, strain-tolerant microstructure that enhances the spallation performance of the deposited coating. The resistance to heat flow through this coating structure is enhanced by the defect matrix in this structure created by the “dissolving” of yttria (the dopant oxide) into zirconia, as well as process-induced porosity. This EB-PVD deposited yttria-stabilized coating provides a “feathery” microstructure that is the result of the presence of sub-grains within the columns of the coating. These sub-grain interface boundaries can be viewed as being low-angle grain boundaries formed by porosity within the columns. It is believed that the boundaries between the sub-grains make the major contribution to the reduced thermal conductivity of the EB-PVD formed thermal barrier coating structure (when compared to the bulk ceramic).
When exposed to higher engine operating temperatures, this “feathery” microstructure begins to sinter. (This sintering process also partially takes place during the deposition of the coating by EB-PVD.) It has been found that this sintering occurs due to diffusion at the grain and sub-grain boundaries caused by the presence of defects in this microstructure. This results in coarsening and collapsing of the original porosity, as well as a reduction of the interface boundary area. These microstructural changes that occur at elevated temperatures enhance the thermal conductivity of the thermal barrier coating, and thus reduce the thermal insulation of the underlying metal substrate. Indeed, this sintering process can increase the thermal conductivity of the thermal barrier coating by as much as 20 to 30%.
While this sintering process is particularly evident in EB-PVD deposited yttria-stabilized zirconia thermal barrier coatings, similar effects can occur in such coatings deposited by plasma spraying. In the case of plasma sprayed yttria-stabilized zirconia thermal barrier coatings, the splat boundaries are the conductivity reducing feature of such coatings. As a result, any sintering that would occur at such boundaries would be undesirable.
Accordingly, it would be desirable to minimize or reduce this sintering process so as to maintain the insulating efficacy of the thermal barrier coating for the life of the coating, especially with regard to coatings formed by EB-PVD techniques. It would be further desirable to be able to modify the chemical composition of yttria- stabilized zirconia-based thermal barrier coating systems to reduce this sintering tendency and thus maintain or improve the reduced thermal conductivity of such coatings.